This invention relates generally to the preparation of useful adhesives and more particularly to the preparation of aromatic polyimide adhesives which have excellent bonding characteristics of and excellent retention of properties at elevated temperatures.
Adhesive bonding is a well known method for joining similar and dissimilar materials. However, the advent of materials, both metallic and nonmetallic, which are capable of withstanding high temperatures, has generated a subsequent need for adhesives for bonding the advanced materials. The increased use of titanium and thermally resistant composites in particular for applications such as in aircraft structural components has led to the investigation of polyimides as base ingredients for adhesive formulations. The aromatic polyimides have the best thermal stability, but do not give satisfactory adhesion of the joints. Some modifications have been made to the polyimides which result in improved adhesive strengths, but those modifications are generally made so that a sacrifice in the long-term thermal stability of the adhesive resin results.
Accordingly, it is an object of this invention to provide a process for preparing and using aromatic polyimides as adhesives for bonding metals, and fiber-reinforced organic resin composites such that high bonding strengths result with no significant loss in thermooxidative stability of the adhesive resin.
A further object of this invention is to provide a process for preparing polyimide adhesives which will retain a high proportion of their adhesive strength when formulated with various additives and when tested at elevated temperatures.